1. Field of the Invention
The invention relates to an alarm apparatus and an alarm method for detecting a fracture of a window pane.
2. Description of the Related Art
A vehicle antitheft apparatus that detects a breaking sound of a window pane via a pane breakage sensor and, on the basis of the detection output, informs that a window pane has been broken is known (see, e.g., Japanese Patent Application Laid-Open Publication No. JP-A-2004-34934). However, since the pane breakage sensor in the foregoing vehicle antitheft apparatus is formed by a microphone, the impact sound detected is exclusively sound waves of the audio frequency band; therefore, there is a risk that besides detecting a breaking sound of a window pane, the sensor may falsely detect a noise from outside the vehicle as a breaking sound of a window pane.
As a solution to the aforementioned problem, a pane fracture detector apparatus is known which includes a plurality of resistors arranged on window panes of a vehicle, and which determines whether or not a window pane has fractured on the basis of a change in the partial voltage value of the electric power source applied to the resistors (see, e.g., Japanese Patent Application Laid-Open Publication No. JP-A-2003-141649).
However, there are cases in which, during a non-alarm state prior to an alarm state where the determination as to whether or not a window pane has fractured begins, an abnormality occurs in the resistors or the like resulting in a disconnection of the resistor of a window pane. In such a case, there is a risk of outputting a false alarm without a fracture of any window pane immediately after the pane fracture detector apparatus has changed from the non-alarm state and to an alarm state.